The present invention relates to improvements in aircraft instruments and, more particularly, to electroluminescent illumination devices for the direct lighting of aircraft instrument dials including alpha-numeric and symbolic indicia thereon, pointers and the like.
A search of the prior art failed to uncover any prior art references which disclose the improvements of the present invention. The following is a listing of the prior art patents uncovered during the search:
______________________________________ U.S. Pat. No. Patentee Issue Year ______________________________________ 2,140,972 Rylsky 1938 2,259,910 Rylsky 1941 3,027,668 Hardesty 1962 3,027,669 Hardesty 1962 3,068,833 Hoffman 1962 3,223,833 Protzmann 1965 3,701,900 Thuler 1972 3,867,623 Widlund 1975 4,138,620 Dickson 1979 4,144,557 Kerr, III et al 1979 4,181,925 Burrows et al 1980 ______________________________________
From U.S. Pat. No. 4,181,925 to Burrows et al, it is known to illuminate aircraft instrument dials using an area of electroluminescent material arranged in a sealed, glass laminate instrument cover so that, when the electroluminescent material is energized, light is emitted to illuminate a selected portion of the instrument. According to Burrows et al, the electroluminescent material includes a layer of light emitting phosphor dispersed in a resin binder which overlies a tin oxide film applied to the glass laminate. The cover assembly is said to require minimum maintenance because it is a sealed assembly.
U.S. Pat. No. 4,138,620 to Dickson teaches the use of electroluminescent sign panels arranged in sealed, transparent polymeric envelopes to provide a moisture barrier. Alternatively, Dickson teaches employing phosphors encapsulated in a moisture barrier film of tin oxide or equivalent material.
Typically, instruments for military, commercial and private aircraft are conventionally lighted by means of one or more low-voltage incandescent lamps, such as disclosed in the above-listed U.S. Pat. Nos. 2,140,972; 2,259,910; and 3,068,833. Those prior art patents suggest various means for improving the uniformity of the illumination provided by the incandescent lamps, such as annular light-conducting elements and the like. However, failure of an incandescent lamp can occur at any time and result in either non-uniform lighting of the instrument or complete loss of lighting.
Most instruments for the present day high performance military aircraft are designed as sealed modular units which are illuminated by several incandescent lamps disposed internally of the unit. If even a single lamp fails, the entire instrument is normally replaced and the instrument with the failed lamp or lamps is returned to the manufacturer for lamp replacement. Such a procedure is expensive and unnecessarily time consuming.